1. Field of the Invention
The present invention relates to a transmission, and more particularly to a transmission designed to achieve six forward speed stages and one reverse speed stages, wherein an input shaft and an output shaft (countershaft) are disposed on respective different axes.
2. Description of the Related Art
Heretofore, as a transmission intended to be disposed on a lateral side of a transversely disposed engine (i.e., transverse engine), a so-called “transverse transmission” has been commonly employed which is designed such that an input shaft and an output shaft (countershaft) are disposed in parallel on respective different axes, and a plurality of speed-change gear sets are disposed between the input shaft and the output shaft (countershaft).
In view of improvement in drive feeling, there exists a strong need for the transverse transmission to further increase the number of speed stages, as with other types of transmissions.
In reality, a design aiming to increase the number of speed stages in the transverse transmission involves an increase in the number of speed-change gears, which leads to an increase in overall length of a combination of an engine and a transmission (i.e., Powertrain) and cause difficulty in adequately laying out the power train in an engine compartment of a vehicle.
Thus, it is necessary to minimize an overall transmission length while increasing the number of speed stages.
From this standpoint, the following transmission has been proposed in U.S. Pat. No. 5,906,132 (hereinafter referred to as “Patent Document 1”).
This transmission comprises an input shaft, and three auxiliary shafts, such as a “first countershaft”, a “second countershaft” and a “reverse shaft”, wherein each of a plurality of differential driving gears (first to third output gears) for driving a differential ring gear (final ring gear) provided in a differential casing on a driving axle is provided on a corresponding one of the auxiliary shafts in such a manner as to partially share a torque transmission path to minimize an overall transmission length.
In the above type of transmission having three auxiliary shafts in addition to an input shaft, how to lay out the increased number of transmission shafts holds a key to downsizing of the transmission.
For example, in cases where a transmission is disposed on a lateral side of a reciprocating engine, the transmission is laid out on a lateral side of a lower portion of the engine in conformity to a position of a crankshaft. Thus, it is necessary to maximally downsize an upper portion of the transmission so as to ensure a space on a lateral side of an upper portion of the engine to provide enhanced flexibility in layout of engine components and vehicle body frames.
From this point of view, in the transmission disclosed in the Patent Document 1 where only two gears, i.e., a fifth-speed driven gear (i.e., a driven gear for a fifth speed stage) and a sixth-speed driven gear (i.e., a driven gear for a sixth speed stage), are provided on the first countershaft disposed at a position above the input shaft, a length of the first countershaft can be reduced to provide an increased space on the lateral side of the upper portion of the engine.
On the other hand, in the transmission disclosed in the Patent Document 1, first-speed and second-speed driven gears (i.e., driven gears for first and second speed stages) to be subjected to a relatively large driving torque are provided on the second countershaft having a relatively large length. This gives rise to a need for increasing a diameter of the second countershaft in order to resist the driving torque. Thus, there is a problem that the second countershaft cannot be downsized.
Moreover, a relatively large support span (i.e., bearing distance) of the second countershaft gives rise to another need for increasing support stiffness of a bearing for supporting the second countershaft to cause a problem about an increase in size of the bearing.
As above, the transmission disclosed in the Patent Document 1 has a problem about difficulty in achieving desirable downsizing thereof.